In hot-sections of gas turbines and other heat engines, high temperature oxidation resistant coatings are necessary to protect engine components and thereby improve engine reliability. Alloyed metallic coatings that form protective, slow growing oxide scales such as alumina (Al2O3) and chromia (Cr2O3) have been designed and used as oxidation and corrosion resistant coatings, thus affording for load-bearing components to have longer service lives. However, metallic coatings typically have a useful temperature capability of less than 1000° C., and may melt above 1350° C.
Lightweight and high temperature capable silicon-base ceramics are desirable and can be used for improving gas turbine engine performance. However, thermal and environmental barrier coatings are critical when employing the ceramic technology. The environmental barrier coatings are a necessity in providing thermal and environmental protection of the silicon-base ceramic components due to the high volatility of silicon-base ceramics in high temperature oxidizing and water vapor containing combustion environments. In order to increase the temperature capability of silicon-base ceramic engine components, thermal and environmental barrier coatings are applied to the component surfaces. Thermal and environmental barrier coatings are relatively thin ceramic layers, generally applied by plasma-spraying or physical vapor deposition techniques, that are used to protect metallic and ceramic components from high temperature gases, water vapor and other oxidants. Such coatings are useful in protecting and extending the service lives of ceramic components exposed to high temperatures, such as jet engine turbine blades, vanes and combustors.
Outer layer thermal barrier coatings composed of zirconia-yttria are well known in the art, wherein the yttria is typically present from 7 to 9 weight percent (wt %) (4 to 5 molar percent). The coatings are generally applied by plasma-spraying or physical vapor deposition, in which melted ceramic particles or vaporized ceramic clouds are deposited onto the surface of a component to be protected, and have been widely used in more advanced engine systems. The thermal barrier coatings are somewhat porous with overall porosities typically in the range of 5 to 20 percent. The porosity serves to reduce the thermal conductivity of the coating below the intrinsic conductivity of the dense ceramic of the same composition.
Beneath an outer layer thermal barrier coating, it is not uncommon for a coating system to have an intermediate layer plus and a bond coat adjacent to the substrate. The intermediate layer can be referred to as an environmental barrier coating. Current state-of-the-art environmental barrier coatings for silicon-base ceramics are based on barium strontium aluminosilicate (BSAS), with a mullite+BSAS intermediate layer, and a silicon bond coat adjacent to the silicon-base ceramics. One problem with current environmental barrier coatings is the limited high temperature stability, and thus capability, of such systems. These barrier coatings have a temperature capability of 1350° C. and below due to the: (1) relatively poor water vapor corrosion resistance of the coating below 1300° C.; and (2) low melting eutectic glass phases resulting from silicon interdiffusion and interface reactions in the mullite+BSAS/silicon system. Therefore, an environmental barrier coating with high water vapor stability and high temperature capability is needed.